USB: f_mtp: Revert Avoid queuing of receive_file_work for 0 length

Commit 76f7005a ("USB: f_mtp: Avoid queuing of receive_file_work
for 0 length") adds a check for 0 length file and avoid queuing in
order to save some milliseconds in mtp file transfers.
However, this approach is correct only for windows system as it
ignores 0 length files but other OS like MAC & Ubuntu do not follow
this logic. They allows 0 length file to be copied which is why mtp
file transfer failure is observed when target is connected to MAC
or Ubuntu.

Fix this by reverting the check for 0 length.

Change-Id: I48559e89afa7a4220bdef3f078e2aa739c402b4b
Signed-off-by: Udipto Goswami <ugoswami@codeaurora.org>